When an object of any shape is moulded, in particular by injection, and a hook field is desired to be provided thereon which will enable the object to be attached, for example via loops of a woven material or other hooks which co-operate with the hook field, an insertion block is made beforehand which is intended to be inserted in the mould for producing the moulded object and comprises in general a field of cavities issuing at its upper surface and having a shape complementary to that of the hooks of the hook field to be formed. This insertion block is then disposed in the mould in which the moulded object will be moulded, the hooks thus being formed at the same time as the moulded object and in one piece therewith.
The insertion blocks known nowadays are formed of plates stacked one against the other, and on the upper edge or field of some of these plates, cut-outs having a shape complementary to that of the hooks are formed, by machining e.g., by spark-machining, laser etching, chemical cutting or the like. The stacked plates are generally formed of two types of plates, i.e., plates whose fields have cut-outs and plates without cut-outs—these latter plates being disposed between two plates with cut-outs to thereby form the cavities, each cavity being formed of one cut-out and the lateral walls of the adjoining plates without cut-outs. However, plates can also be stacked all having cut-outs on their field, by offsetting the cut-outs formed in two successive plates such that the part of one plate between two of its successive cut-outs forms a lateral wall of the cavities of the adjoining plates.
Once the plates are stacked one on top of the other, these are pressed together by jaw-type systems and/or are welded at the periphery thereof to keep them together, wherein a combination of these methods is also possible.
These insertion blocks of the prior art have many drawbacks:
                when they are placed in the mould for forming the moulded object on which they are intended to permit the addition of the hook field, the plates which are clamped together or are welded together at their ends might slightly separate from each other under the effect of the injection pressure of the thermoplastic material allowing the passage of the thermoplastic material between two successive plates. Upon exiting the mould, the hook field formed on the moulded object thus has a burr formed by a sort of wall, often with a greater height than the hooks and extending over substantially the entire width of the hook field, wherein this burr might make the hook field unusable or at least non-compliant for fixation to, for example, another hook field or to loops of a woven material. This also makes the moulded object as a whole unusable, which object must be scrapped. Furthermore, the insertion block would then have to be repaired, i.e., the process of producing the moulded object would have to be stopped and it would have to be ensured that the plates forming the insertion block are better clamped together. This, of course, results in costs in terms of time, personnel and productivity.        
On the other hand, these insert blocks of the prior art make it difficult, owing to the need to clamp and keep the plates against each other, to produce inserts with a small thickness, e.g., less than 10 mm.
Furthermore, the current inserts require a device to keep the plates clamped together which means that the introduction of the insert and of this clamping/holding device into the mould requires a lot of space, resulting in particular in a used surface area which is much greater than the surface area taken up by the hook field formed by the insert.